For fluid treatment systems using exposition to high-energy radiation, especially high-energy ultraviolet radiation, there are various applications such as, for example, sterilization, curing of lacquers and synthetic resins, flue-gas purification, destruction and synthesis of special chemical compounds.
Further applications for the device according to the invention have to do with water and wastewater technology where polluted water is the fluid to be treated. Examples of treatment of this kind that may be mentioned are a) disinfection b) breakdown of pollutants and dyes and removal of odor and c) destroying solvent residues present in water.
The device according to the invention may also be used for the treatment of other liquids and solvents besides water, as well as for the treatment of gases.
These applications are generally based on photo-physical or photochemical processes. For use in any of these processes, the radiation source must be able to effectively irradiate the process fluid with high-energy radiation, such as (V) UV radiation, since it is the (V) UV photon that starts the desired photo-physical or photochemical reaction. Also, the wavelength of the radiation should be tuned very precisely to the intended process.
Therefore, fluid treatment by photophysical or photochemical processes requires radiation sources that provide high intensity UV radiation, preferably UV radiation, emitting specific radiation in a narrow wavelength range.
These requirements are excellently met by excimer lamps, provided among other things in the form of dielectric barrier discharge (DBD) lamps. In DBD lamps, the electrical energy is coupled by capacitive means to the discharge volume. A DBD lamp can be realized when a high voltage is applied across a gas-filled discharge gap, which is separated from electrodes by at least one dielectric barrier. Dielectric barriers include, for instance, glass or quartz. Due to the nature of the discharge to generate non-thermal plasmas at atmospheric gas pressure, excited diatomic molecules (excimers) are produced, when rare gases, or mixtures of rare gases and halogens are used as the discharge gas. The excimers emit high-energy radiation in the ultraviolet spectral range when they decay.
A dielectric barrier discharge lamp has various advantages which neither a conventional mercury low-pressure lamp nor a conventional high-pressure arc discharge lamp have; for example, emission of ultraviolet radiation with short waves, such as 172 nm, 222 nm, and 308 nm, and at the same time generation of light with individual high-efficiency wavelengths which are roughly like line spectra, are achieved. The emission wavelength depends on the type of gas filling, e.g. xenon filling provides emission at 172 nm.
For fluid treatment systems typically a dielectric barrier discharge lamp of the double tube type is used, which consists of an inner tube and an outer tube.
That is to say, the wall of the inner tube spans a cavity, which likewise forms a type of turned-in section in the discharge vessel, in which one or more internal electrodes are located.
For safety, the internal electrode is located on the high-voltage side and the electrode located in the outer tube is located on the grounded side. In this manner, the internal electrodes are protected against unintentional access.
There is a tendency to use the minimum number of lamps possible and run them at high power density.
As a lamp run at a high power density has a higher internal temperature than a lamp run at a lower power, there is the problem that during use the electrodes and discharge gas can be overheated. Overheating the electrodes alters the wavelength of the emitted radiation, reduces the lamp efficiency and can lead to degradation of the electrodes and the dielectric material, reducing the useful lifetime of the lamp.
As a result of temperature differences within the discharge vessel, there may also be differences in the wavelengths of the emitted radiation generated by the lamp. This results in a broader spectrum of emitted radiation, which can have undesired consequences on fluid treatment processing.
Thus, a significant problem in the art is how to overcome these differences in temperatures locally within the devices, and how to keep the lamp cool enough to efficiently generate UV or VUV radiation.
From U.S. Pat. No. 5,834,784 a lamp is known that is constructed in the form of two concentric quartz cylinders sealed together at their ends with the excimer gas fill between the cylinders. Cooling liquid is pumped through the central region inside the inner quartz cylinder where an electrically conductive pipe which is not in contact with the inner cylinder is used to supply this cooling liquid. Although it is not in contact with the inner quartz cylinder, this central pipe also acts as the high-voltage electrode. A cable attaches the central pipe to a high-voltage AC power source, but this high-voltage electrode is electrically insulated from the source of cooling liquid by a suitably long length of electrically insulated tubing which also supplies the cooling liquid.
Under the rough conditions in a water treatment plant, such lamp may provide a safety hazard, if the electrically insulated tubing is not carefully protected against unintended breakage.
Thus the principal object of the present invention is to create an internal cooling for a radiation source according to the prior art, which does not impair the operational safety of the fluid treatment system, but which nevertheless makes possible an efficient cooling of the inner tube of the radiation source.